It is known that steel sheet immersed in a hot-dip plating bath of zinc containing an appropriate amount of Al and Mg to plate the steel sheet with this alloy exhibits excellent corrosion resistance. Because of this, various avenues of research and development have been pursued regarding this type of Zn--Al--Mg-system. Up to now, however, no case of a plated steel sheet of this system having achieved commercial success as an industrial product has been seen.
The specification of U.S. Pat. No. 3,505,043, for example, teaches a hot-dip Zn--Al--Mg plated steel sheet with excellent corrosion resistance using a hot-dip plating bath composed of Al: 3-17 wt. %, Mg: 1-5 wt. % and the remainder of Zn. This was followed by proposals set out in, for example, JPB-64-8702, JPB-64-11112 and JPA-8-60324 for improving corrosion resistance and productivity by incorporating various addition elements in the basic bath composition of this type, regulating the production conditions, and the like.